When a lubricating medium of an expander has become mixed with a working medium circulating around a closed circuit of a Rankine cycle system, the amount of lubricating medium in the expander becomes insufficient, thus degrading the efficiency of the expander or causing seizing. Japanese Utility Model Publication No. 61-8170 discloses a gas/liquid separator for separating a lubricating medium from a working medium and returning it to an expander.
There is also known from Japanese Patent Application Laid-open No. 63-156508 a so-called coalescer type oil/water separating filter in which, by supplying a mixture of oil and water to an ultrafine fiber filter, oil droplets attached to the fiber become coarser and thus separate from the water by virtue of the difference in specific gravity between the oil and water, or water droplets attached to the fiber become coarser and thus separate from the oil by virtue of the difference in specific gravity between water and the oil.
However, in the Rankine cycle system disclosed in Japanese Utility Model Publication No. 61-8170, since the mixture of the working medium and the lubricating medium circulates in the closed circuit, there is a possibility that the lubricating medium in the working medium circulating in the closed circuit might gasify due to heat, thus affecting the performance and the durability of the Rankine cycle system. Furthermore, since a mixture of liquid-phase working medium, gas-phase working medium, and lubricating medium is supplied from a boiler to the gas/liquid separator and, moreover, the gas/liquid separator has a structure in which the lubricating medium is separated by gravity, there is the problem that it is impossible to prevent the liquid-phase working medium from becoming mixed with the lubricating medium.